This invention relates to pressure sensors for sensing the pressure of a fluid (liquid or gas) with a variable capacitor.
One known type of pressure sensor uses a variable capacitor to sense the pressure of a received fluid (liquid or gas). Such sensors, such as a BARATRON.RTM. Absolute Pressure Transmitters (BARATRON is a registered trademark of MKS Instruments, Inc. of Andover, Mass.), are often used in industrial applications, e.g., to measure the pressure of fluids in semiconductor processing equipment.
A known design for such a sensor has a housing that defines an interior chamber and an inlet for receiving a fluid whose pressure is to be sensed. First and second conductive electrodes are mounted in the housing, generally in parallel, and are spaced apart by a small gap to form a parallel plate capacitor. The first electrode is fixed relative to the housing, while the second electrode is movable relative to the first electrode in response to the received fluid. In one implementation, the first electrode is formed on a ceramic support disk with thick film deposition techniques, and the second electrode is a diaphragm, typically made of a metal, such as an alloy of nickel, chromium, and iron, sold under the name INCONEL.RTM. (a registered trademark of Inco Alloy International of Huntington, W. Va.).
The movable second electrode is typically clamped at its periphery and extends across the width of the sensor to define first and second chambers within the interior. The first chamber has a reference inlet by which a known reference pressure can be established, e.g., zero pressure. The second chamber has an inlet for receiving the fluid to be sensed, causing a central portion of the diaphragm to flex in response to changes in the pressure of the fluid. This flexing movement causes the gap between the electrodes to change. An electrical signal is provided to the first electrode (the movable second electrode is typically grounded) so that the change in capacitance between the first and second electrodes can be sensed and related to the pressure of the received fluid.
For the pressure to be sensed meaningfully and in a way that can be accurately resolved, the diaphragm must be large enough so that it can flex sufficiently so that the change in the gap, and hence the change in capacitance, can be sensed with sufficient resolution. As the diaphragm is made smaller and smaller, the gap between the electrodes must be made smaller. In mathematical terms, it is well known that C=eA/d for a parallel plate capacitor, where C is the capacitance, e is a constant based on the material between the plates (e=1 for a vacuum), A is the common area of the plates, and d is the gap. This means that the change in capacitance with respect to a change in the gap, dC/dd=-eAd.sup.-2. As this equation indicates, the smaller A is, the harder it is to accurately detect changes in dC/dd. Consequently, with a small common area, there must be a very small gap to permit accurate sensing. In that case, however, it is very important to control the gap in an accurate and repeatable manner, but such control is difficult with a small gap.
Accordingly, while it would be desirable to be able to make such sensors smaller in order to reduce space and costs, it can be difficult to control the gap in an accurate and repeatable manner.